1. Field of the Invention
This invention relates to permeable inorganic membranes. More particularly, this invention relates to permeable inorganic membranes stabilized to resist attack by acidic or alkaline solutions by coating the surface with a substantially monomolecular layer of a phosphoric acid ester. 2. Description of the Related Art
Various inorganic membranes made from metals, inorganic polymers, and ceramics have been proposed for liquid and gas separations applications. The interest in utilizing such membranes in separations has increased since the advent of consistent quality, commercially available ceramic membranes with narrow size distributions. Inorganic membranes exhibit unique physical and chemical properties that are only partially or not shown by organic membranes. For example, they can be used at significantly higher temperatures, have better structural stability without the problems of swelling or compaction, generally can withstand more harsh chemical environments, are not subject to microbiological attack, and can be backflushed, steam sterilized, or autoclaved. Currently, microporous stainless steel, silver, and ceramic membranes such as alumina, zirconia, and glass are available commercially.
While such inorganic membranes such as the ceramic membranes offer distinct advantages over the more widely prevalent organic polymer membranes previously used, the pores of the inorganic membranes may also become blocked during usage. Pore blockage causes decreased permeability, flux, and efficiency of the membrane. Higher inlet pressures are then required to maintain a given flow rate. To restore membrane performance by opening pores, strongly acidic solutions (e.g., 2 wt. % nitric acid), or strongly basic solutions (e.g., 2 wt. % sodium hydroxide) are used to dissolve the solid materials clogging the pores. However, most inorganic membrane materials, particularly ceramic materials such as alumina and silica, are also attacked by such aggressive solutions. Enlargement of the pore dimensions or even partial dissolution of the entire membrane material can result. In both cases the membrane may be rendered useless for the intended application.
It would, therefore, be advantageous to provide a permeable inorganic membrane which would provide the previously discussed advantages over organic membranes yet also exhibit greater chemical stability over a wide range of pH to permit processing of the membrane to dissolve blockages without attack and destruction of the ceramic membrane itself.
In Wieserman et al U.S. Ser. No. 946,870, cross-reference to which is hereby made, there is described a method for treating a metal oxide/hydroxide substrate to provide a monomolecular layer of a phosphonic or phosphinic acid thereon to stabilize the surface of the substrate over a wide pH range. In Martin et al U.S. Ser. No. 023,312, cross-reference to which is also made, there is described a method for treating a metal oxide/hydroxide substrate to provide a monomolecular layer of a phosphoric acid ester thereon to stabilize the surface of the substrate over a wide pH range.